This is a selection of command-line tips that I've found useful over the years when working on Linux. It's organized as a journey, from command-line novice to expert. Some tips are elementary, and some sophisticated enough very few people know them.
The goal is broad coverage of important tips that a technical user will find useful or time-saving. It's a bit long, and users certainly don't need to know all of them, but I've done my best to review that each item is worth reading in terms of projected time savings, if you use Linux heavily. It's written for Linux, but many items apply equally to MacOS. The items are intentionally brief. For more information on a command, try man, aptitude/yum, or Google.
The goal is broad coverage of important tips that a technical user will find useful or time-saving. It's a bit long, and users certainly don't need to know all of them, but I've done my best to review that each item is worth reading in terms of projected time savings, if you use Linux heavily. It's written for Linux, but many items apply equally to MacOS. The items are intentionally brief. For more information on a command, try man, apt-get/yum, or Google.
I originally wrote much of this as an answer [on Quora](http://www.quora.com/What-are-some-time-saving-tips-that-every-Linux-user-should-know/answer/Joshua-Levy), but given the interest there. it seems it's worth moving the list to Github, where people more talented than I can readily suggest improvements. If you see an error or something that could be better, please submit a PR!
## Basics
- Learn basic Bash. Actually, read the whole bash man page; it's pretty easy to follow and not that long. Alternate shells can be nice, but bash is powerful and always available (learning mainly zsh or tcsh restricts you in many situations).
- Learn basic Bash. Actually, type `man bash` and at least skim the whole thing; it's pretty easy to follow and not that long. Alternate shells can be nice, but bash is powerful and always available (learning mainly zsh, fish, etc. restricts you in many situations).
- Learn vim. There's really no competition for random Linux editing (even if you use Emacs or Eclipse most of the time).
- Learn vim. There's really no competition for random Linux editing (even if you use Emacs, a big IDE, or a modern hipster editor most of the time).
- Know ssh, and the basics of passwordless authentication, via ssh-agent, ssh-add, etc.
- Know ssh, and the basics of passwordless authentication, via `ssh-agent`, `ssh-add`, etc.
- Be familiar with bash job management: &, Ctrl-Z, Ctrl-C, jobs, fg, bg, kill, etc.
- Be familiar with bash job management: &, Ctrl-Z, Ctrl-C, `jobs`, `fg`, `bg`, `kill`, etc.
- Basic file management: ls and ls -l (in particular, learn what every column in "ls -l" means), less, head, tail and tail -f, ln and ln -s (learn the differences and advantages of hard versus soft links), chown, chmod, du (for a quick summary of disk usage: du -sk *), df, mount.
- Basic file management: `ls` and `ls -l` (in particular, learn what every column in `ls -l` means), `less`, `head`, `tail` and `tail -f`, `ln` and `ln -s` (learn the differences and advantages of hard versus soft links), `chown`, `chmod`, `du` (for a quick summary of disk usage: `du -sk *`), `df`, `mount`.
- Basic network management: ip or ifconfig, dig.
- Basic network management: `ip` or `ifconfig`, `dig`.
- Know regular expressions well, and the various flags to grep/egrep. The -o, -A, and -B options are worth knowing.
- Know regular expressions well, and the various flags to `grep`/`egrep`. The `-o`, `-A`, and `-B` options are worth knowing.
- Learn to use apt-get or yum (depending on distro) to find and install packages.
- Learn to use `apt-get` or `yum` (depending on distro) to find and install packages.
## Everyday use
@ -38,23 +38,25 @@ I originally wrote much of this as an answer [on Quora](http://www.quora.com/Wha
- If you are halfway through typing a command but change your mind, hit Alt-# to add a # at the beginning and enter it as a comment (or use Ctrl-A, #, enter). You can then return to it later via command history.
- Use xargs (or parallel). It's very powerful. Note you can control how many items execute per line (-L) as well as parallelism (-P). If you're not sure if it'll do the right thing, use xargs echo first. Also, -I{} is handy. Examples:
- Use `xargs` (or `parallel`). It's very powerful. Note you can control how many items execute per line (-L) as well as parallelism (-P). If you're not sure if it'll do the right thing, use xargs echo first. Also, -I{} is handy. Examples:
```
find . -name \*.py | xargs grep some_function
cat hosts | xargs -I{} ssh root@{} hostname
```
- pstree -p is a helpful display of the process tree.
- `pstree -p` is a helpful display of the process tree.
- Use pgrep and pkill to find or signal processes by name (-f is helpful).
- Use `pgrep` and `pkill` to find or signal processes by name (`-f` is helpful).
- Know the various signals you can send processes. For example, to suspend a process, use kill -STOP [pid]. For the full list, see man 7 signal
- Know the various signals you can send processes. For example, to suspend a process, use `kill -STOP [pid]`. For the full list, see man 7 signal
- Use nohup or disown if you want a background process to keep running forever.
- Check what processes are listening via netstat -lntp. See also lsof.
- In bash scripts, use set -x for debugging output. Use set -e to abort on errors. - Consider using set -o pipefail as well, to be strict about errors (though this topic is a bit subtle). For more involved scripts, also use trap.
- In bash scripts, use set -x for debugging output.
- In bash scripts, use strict modes whenever possible. Use `set -e` to abort on errors. Use `set -o pipefail` as well, to be strict about errors (though this topic is a bit subtle). For more involved scripts, also use `trap`.
- In bash scripts, subshells (written with parentheses) are convenient ways to group commands. A common example is to temporarily move to a different working directory, e.g.
```
@ -63,24 +65,24 @@ I originally wrote much of this as an answer [on Quora](http://www.quora.com/Wha
# continue in original dir
```
- In bash, note there are lots of kinds of variable expansion. Checking a variable exists: ${name:?error message}. For example, if a bash script requires a single argument, just write input_file=${1:?usage: $0 input_file}. Arithmetic expansion: i=$(( (i + 1) % 5 )). Sequences: {1..10}. Trimming of strings: ${var%suffix} and ${var#prefix}. For example if var=foo.pdf, then echo ${var%.pdf}.txt prints "foo.txt".
- In bash, note there are lots of kinds of variable expansion. Checking a variable exists: `${name:?error message}`. For example, if a bash script requires a single argument, just write `input_file=${1:?usage: $0 input_file}`. Arithmetic expansion: `i=$(( (i + 1) % 5 ))`. Sequences: `{1..10}`. Trimming of strings: `${var%suffix}` and `${var#prefix}`. For example if `var=foo.pdf`, then `echo ${var%.pdf}.txt` prints `foo.txt`.
- The output of a command can be treated like a file via <(some command). For example, compare local /etc/hosts with a remote one:
- The output of a command can be treated like a file via `<(some command)`. For example, compare local /etc/hosts with a remote one:
```
diff /etc/hosts <(ssh somehost cat /etc/hosts)
diff /etc/hosts <(ssh somehost cat /etc/hosts)
```
- Know about "here documents" in bash, as in cat <<EOF ....
- Know about "here documents" in bash, as in `cat <<EOF ...`.
- In bash, redirect both standard output and standard error via: some-command >logfile 2>&1. Often, to ensure a command does not leave an open file handle to standard input, tying it to the terminal you are in, it is also good practice to add "</dev/null".
- In Bash, redirect both standard output and standard error via: `some-command >logfile 2>&1`. Often, to ensure a command does not leave an open file handle to standard input, tying it to the terminal you are in, it is also good practice to add `</dev/null`.
- Use man ascii for a good ASCII table, with hex and decimal values.
- Use `man ascii` for a good ASCII table, with hex and decimal values.
- On remote ssh sessions, use screen or dtach to save your session, in case it is interrupted.
- On remote ssh sessions, use `screen` or `dtach` to save your session, in case it is interrupted.
- In ssh, knowing how to port tunnel with -L or -D (and occasionally -R) is useful, e.g. to access web sites from a remote server.
- It can be useful to make a few optimizations to your ssh configuration; for example, this .ssh/config contains settings to avoid dropped connections in certain network environments, not require confirmation connecting to new hosts, forward authentication, and use compression (which is helpful with scp over low-bandwidth connections):
- It can be useful to make a few optimizations to your ssh configuration; for example, this `~/.ssh/config` contains settings to avoid dropped connections in certain network environments, not require confirmation connecting to new hosts, forward authentication, and use compression (which is helpful with scp over low-bandwidth connections):
- To get the permissions on a file in octal form, which is useful for system configuration but not available in "ls" and easy to bungle, use something like
- To get the permissions on a file in octal form, which is useful for system configuration but not available in `ls` and easy to bungle, use something like
```
stat -c '%A %a %n' /etc/timezone
```
## Data processing
- To convert HTML to text: lynx -dump -stdin
- To convert HTML to text: `lynx -dump -stdin`
- If you must handle XML, xmlstarlet is old but good.
- If you must handle XML, `xmlstarlet` is old but good.
- For JSON, use jq.
- For JSON, use `jq`.
- For Amazon S3, s3cmd is convenient (albeit immature, with occasional misfeatures).
- For Amazon S3, `s3cmd` is convenient (albeit immature, with occasional misfeatures) and `s4cmd` is faster.
- Know about sort and uniq (including uniq's -u and -d options). See one-liners below.
- Know about `sort` and `uniq` (including uniq's `-u` and `-d` options). See one-liners below.
- Know about cut, paste, and join to manipulate text files. Many people use cut but forget about join.
- Know about `cut`, `paste`, and `join` to manipulate text files. Many people use `cut` but forget about `join`.
- Know that locale affects a lot of command line tools, including sorting order and performance. Most Linux installations will set LANG or other locale variables to a local setting like US English. This can make sort or other commands run many times slower. (Note that even if you use UTF-8 text, you can safely sort by ASCII order for many purposes.) To disable slow i18n routines and use traditional byte-based sort order, use export LC_ALL=C (in fact, consider putting this in your .bashrc).
- Know that locale affects a lot of command line tools, including sorting order and performance. Most Linux installations will set `LANG` or other locale variables to a local setting like US English. This can make sort or other commands run many times slower. (Note that even if you use UTF-8 text, you can safely sort by ASCII order for many purposes.) To disable slow i18n routines and use traditional byte-based sort order, use `export LC_ALL=C` (in fact, consider putting this in your `~/.bashrc`).
- Know basic awk and sed for simple data munging. For example, summing all numbers in the third column of a text file: awk '{ x += $3 } END { print x }'. This is probably 3X faster and 3X shorter than equivalent Python.
- Know basic `awk` and `sed` for simple data munging. For example, summing all numbers in the third column of a text file: `awk '{ x += $3 } END { print x }'`. This is probably 3X faster and 3X shorter than equivalent Python.
- To replace all occurrences of a string in place, in one or more files:
- Use shuf to shuffle or select random lines from a file.
- Use `shuf` to shuffle or select random lines from a file.
- Know sort's options. Know how keys work (-t and -k). In particular, watch out that you need to write -k1,1 to sort by only the first field; -k1 means sort according to the whole line.
- Know `sort`'s options. Know how keys work (`-t` and `-k`). In particular, watch out that you need to write `-k1,1` to sort by only the first field; `-k1` means sort according to the whole line.
- Stable sort (sort -s) can be useful. For example, to sort first by field 2, then secondarily by field 1, you can use sort -k1,1 | sort -s -k2,2
- Stable sort (`sort -s`) can be useful. For example, to sort first by field 2, then secondarily by field 1, you can use `sort -k1,1 | sort -s -k2,2`
- If you ever need to write a tab literal in a command line in bash (e.g. for the -t argument to sort), press Ctrl-V <tab> or write $'\t' (the latter is better as you can copy/paste it).
- For binary files, use hd for simple hex dumps and bvi for binary editing.
- For binary files, use `hd` for simple hex dumps and `bvi` for binary editing.
- Also for binary files, strings (plus grep, etc.) lets you find bits of text.
- Also for binary files, `strings` (plus `grep`, etc.) lets you find bits of text.
- To convert text encodings, try iconv. Or uconv for more advanced use; it supports some advanced Unicode things. For example, this command lowercases and removes all accents (by expanding and dropping them):
- To convert text encodings, try `iconv`. Or `uconv` for more advanced use; it supports some advanced Unicode things. For example, this command lowercases and removes all accents (by expanding and dropping them):
- To split files into pieces, see split (to split by size) and csplit (to split by a pattern).
- To split files into pieces, see `split` (to split by size) and `csplit` (to split by a pattern).
## System debugging
- For web debugging, curl and curl -I are handy, and/or their wget equivalents.
- For web debugging, `curl` and `curl -I` are handy, or their wget equivalents, or the more modern `httpie`.
- To know disk/cpu/network status, use iostat, netstat, top (or the better htop), and (especially) dstat. Good for getting a quick idea of what's happening on a system.
- To know disk/cpu/network status, use `iostat`, `netstat`, `top` (or the better `htop`), and (especially) `dstat`. Good for getting a quick idea of what's happening on a system.
- To know memory status, run and understand the output of free and vmstat. In particular, be aware the "cached" value is memory held by the Linux kernel as file cache, so effectively counts toward the "free" value.
- To know memory status, run and understand the output of `free` and `vmstat`. In particular, be aware the "cached" value is memory held by the Linux kernel as file cache, so effectively counts toward the "free" value.
- Java system debugging is a different kettle of fish, but a simple trick on Sun's and some other JVMs is that you can run kill -3 <pid> and a full stack trace and heap summary (including generational garbage collection details, which can be highly informative) will be dumped to stderr/logs.
- Java system debugging is a different kettle of fish, but a simple trick on Oracle's and some other JVMs is that you can run `kill -3 <pid>` and a full stack trace and heap summary (including generational garbage collection details, which can be highly informative) will be dumped to stderr/logs.
- Use mtr as a better traceroute, to identify network issues.
- Use `mtr` as a better traceroute, to identify network issues.
- For looking at why a disk is full, ncdu saves time over the usual commands like "du -sk *".
- For looking at why a disk is full, `ncdu` saves time over the usual commands like `du -sk *`.
- To find which socket or process is using bandwidth, try iftop or nethogs.
- To find which socket or process is using bandwidth, try `iftop` or `nethogs`.
- The ab tool (comes with Apache) is helpful for quick-and-dirty checking of web server performance. For more complex load testing, try siege.
- The `ab` tool (comes with Apache) is helpful for quick-and-dirty checking of web server performance. For more complex load testing, try `siege`.
- For more serious network debugging, wireshark or tshark.
- For more serious network debugging, `wireshark` or `tshark`.
- Know strace and ltrace. These can be helpful if a program is failing, hanging, or crashing, and you don't know why, or if you want to get a general idea of performance. - Note the profiling option (-c), and the ability to attach to a running process (-p).
- Know about `strace` and `ltrace`. These can be helpful if a program is failing, hanging, or crashing, and you don't know why, or if you want to get a general idea of performance. Note the profiling option (`-c`), and the ability to attach to a running process (`-p`).
- Know about ldd to check shared libraries etc.
- Know about `ldd` to check shared libraries etc.
- Know how to connect to a running process with gdb and get its stack traces.
- Know how to connect to a running process with `gdb` and get its stack traces.
- Use /proc. It's amazingly helpful sometimes when debugging live problems. Examples: /proc/cpuinfo, /proc/xxx/cwd, /proc/xxx/exe, /proc/xxx/fd/, /proc/xxx/smaps.
- Use `/proc`. It's amazingly helpful sometimes when debugging live problems. Examples: `/proc/cpuinfo`, `/proc/xxx/cwd`, `/proc/xxx/exe`, `/proc/xxx/fd/`, `/proc/xxx/smaps`.
- When debugging why something went wrong in the past, sar can be very helpful. It shows historic statistics on CPU, memory, network, etc.
- When debugging why something went wrong in the past, `sar` can be very helpful. It shows historic statistics on CPU, memory, network, etc.
- For deeper systems and performance analyses, look at stap (systemtap) and perf.
- For deeper systems and performance analyses, look at `stap` (systemtap) and `perf`.
- Confirm what Linux distribution you're using (works on most distros): "lsb_release -a"
- Confirm what Linux distribution you're using (works on most distros): `lsb_release -a`
- Use dmesg whenever something's acting really funny (it could be hardware or driver issues).
- Use `dmesg` whenever something's acting really funny (it could be hardware or driver issues).
## One-liners
- It is remarkably helpful sometimes that you can do set intersection, union, and difference of text files via sort/uniq. Suppose a and b are text files that are already uniqued. This is fast, and works on files of arbitrary size, up to many gigabytes. (Sort is not limited by memory, though you may need to use the -T option if /tmp is on a small root partition.) See also the note about LC_ALL above.
- It is remarkably helpful sometimes that you can do set intersection, union, and difference of text files via `sort`/`uniq`. Suppose `a` and `b` are text files that are already uniqued. This is fast, and works on files of arbitrary size, up to many gigabytes. (Sort is not limited by memory, though you may need to use the `-T` option if `/tmp` is on a small root partition.) See also the note about `LC_ALL` above.
- If want to see sizes/dates on a tree of files, this is like a recursive "ls -l" but is easier to read than "ls -lR":
- If want to see sizes/dates on a tree of files, this is like a recursive `ls -l` but is easier to read than `ls -lR`:
```
find . -type f -ls
```
- Use xargs or parallel for doing many things. Note you can control how many items execute per line (-L) as well as parallelism (-P). If you're not sure if it'll do the right thing, use xargs echo first. Also, -I{} is handy. Examples:
- Use `xargs` or `parallel` whenever you can. Note you can control how many items execute per line (`-L`) as well as parallelism (`-P`). If you're not sure if it'll do the right thing, use xargs echo first. Also, `-I{}` is handy. Examples:
```
find . -name \*.py | xargs grep some_function
cat hosts | xargs -I{} ssh root@{} hostname
```
- Say you have a text file, like a web server log, and a certain value that appears on some lines, such as an acct_id parameter that is present in the URL. If you want a tally of how many requests for each acct_id:
- Say you have a text file, like a web server log, and a certain value that appears on some lines, such as an `acct_id` parameter that is present in the URL. If you want a tally of how many requests for each `acct_id`:
Joshua Levy
10 years ago