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 bit of a journey, from command-line novice to expert. Some tips are elementary, and some sophisticated enough 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.
I originally wrote this list 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 perhaps it's worth moving it to Github, where people more talented than I can more easily suggest improvements. If you see an error or something that could be better, please submit a PR! :)
The items are intentionally brief. For more information on a command, try man, aptitude/yum, or Google.
## 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).
- 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.
- In bash, use Ctrl-W to kill the last word, and Ctrl-U to kill the line. See man readline for default keybindings in bash. There are a lot. For example Alt-. cycles through prevous arguments, and Alt-* expands a glob.
- 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:
find . -name \*.py | xargs grep some_function
cat hosts | xargs -I{} ssh root@{} hostname
- pstree -p is a helpful display of the process tree.
- 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, subshells (written with parentheses) are convenient ways to group commands. A common example is to temporarily move to a different working directory, e.g.
# do something in current dir
(cd /some/other/dir; other-command)
# 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".
- 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)
- 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".
- 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):
TCPKeepAlive=yes
ServerAliveInterval=15
ServerAliveCountMax=6
StrictHostKeyChecking=no
Compression=yes
ForwardAgent=yes
- 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
- 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 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.
- 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).
- 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 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.
- 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.
- 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).
- 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.
- 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.
cat a b | sort | uniq > c # c is a union b
cat a b | sort | uniq -d > c # c is a intersect b
cat a b b | sort | uniq -u > c # c is set difference a - b
- Summing all numbers in the third column of a text file (this is probably 3X faster and 3X less code than equivalent Python):
awk '{ x += $3 } END { print x }' myfile
- 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. 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
- 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:
