Moving Around the Shell: ls, cd, find and Friends

Almost all server work starts in a shell. This post covers the small set of commands and shortcuts that make up most of a working day, plus the habits that prevent the classic mistakes (deleting the wrong directory, running a command on the wrong server).

Versions in use on this date. Ubuntu 18.04.2 LTS ships bash 4.4 and GNU coreutils 8.28. CentOS 7.6 has bash 4.2, and everything shown here works on both.

What happens when you press Enter

flowchart TB
  K[You type a line] --> RL[Readline<br/>editing and history]
  RL --> EX[Expansion<br/>variables globs tilde]
  EX --> LK{Command lookup}
  LK -->|alias or function| AL[Run alias or function]
  LK -->|builtin like cd| BI[Run inside the shell]
  LK -->|file in PATH| EXE[Fork and exec program]
  AL --> OUT[Output and exit code in $?]
  BI --> OUT
  EXE --> OUT

The shell expands *, ~ and $VAR before the program sees them. That is why rm *.log removes whatever the glob matched at that moment, and why quoting matters.

Moving and looking

pwd                      # where am I
ls -lah                  # long, all files, human sizes
ls -lt | head            # newest first
cd /var/log              # absolute path
cd -                     # back to the previous directory
cd                       # home
pushd /etc; popd         # directory stack
tree -L 2 /etc/nginx     # (apt install tree) quick overview

Finding things

find /var/log -name '*.log' -size +100M          # big logs
find /home -type f -mtime -1                     # changed in last 24 hours
find /tmp -type f -mtime +7 -print               # preview before deleting
find /tmp -type f -mtime +7 -delete              # then delete
locate nginx.conf                                # fast, uses a database (sudo updatedb)
grep -rn "listen 80" /etc/nginx/                 # search file contents

Always run find ... -print first and only add -delete once the list looks right.

Copy, move, delete safely

cp -a source/ backup/        # archive mode keeps permissions and times
mv old.conf old.conf.bak     # rename is a move
rm -i file                   # asks first
mkdir -p a/b/c               # create parents
ls -ld /path/to/dir          # check before: rm -rf /path/to/dir

There is no recycle bin. rm -rf with a typo in a variable (rm -rf $DIR/, with DIR empty) is a well-known way to wipe a server, so use set -u in scripts and read the line twice.

Viewing files

less /var/log/syslog         # q to quit, /text to search, G for end, F to follow
head -n 20 file; tail -n 50 file
tail -f /var/log/auth.log    # follow new lines, Ctrl-C to stop
wc -l file                   # count lines
file mystery.bin             # what type is it

Shortcuts worth learning first

Keys	Effect
Tab (twice)	Complete / list matches
Ctrl-R	Search command history
Ctrl-A / Ctrl-E	Start / end of line
Ctrl-W / Ctrl-U	Delete word / to line start
Ctrl-L	Clear screen
!!	Repeat last command (sudo !! after forgetting sudo)
Alt-.	Last argument of the previous command

Prompt and aliases

Make the prompt show user, host and directory so you never type on the wrong server:

# ~/.bashrc
PS1='\u@\h:\w\$ '
alias ll='ls -lah'
alias grep='grep --color=auto'
HISTSIZE=10000
HISTCONTROL=ignoredups:ignorespace    # a leading space keeps a command out of history

Run source ~/.bashrc to load changes. Use man ls or ls --help before searching the web; the manual is already on the machine.