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Unix

Input/Output Redirection

  • >: put the standard output into the file
  • >>: append the standard output to the file
  • <: read the standard input from the file

Processes

  • Find the limits of a process
cat /proc/<PID>/limits
  • Find a PID by name
ps aux | grep name
# Or
pgrep name
  • Find all processes for the current group
ps -ag
  • Run a process that is not attached to the terminal
nohup <command> &

Filesystem

  • Display file content in octal, decimal or hexadecimal format
od -cb <filename>

Permissions

mode octal file directory
r 1 read read
w 2 write edit/delete
x 4 execute search
  • A file has a set of permission associated with it
$ ls -l /etc/passwd
-rw-r--r-- root root
# group permission (-rw-) | user permission (r--) | other permission (r--) | owener | group
  • A directory is prefixed with d
$ ls -ld /
drwxr-xr-x 26 root root
  • Allow all users to read and execute the file
chmod a+rx file
  • Allow the user who owns the file to execute it
chmod u+x file
  • Turn off write permission for the group
chmod g-w file
  • If a directory is writable, anyone can remove/edit files regardless of the persmission on the files themselves

Inodes

Administrative information is stored in the inode along with its location on disc, how long it is. Symbolic links point to the underlying inode of the file.

$ ln old_file new_file
$ ls -li
7105506 -rw-rw-r-- 2 chouffe chouffe 59 Jun 15 13:29 new_file
7105506 -rw-rw-r-- 2 chouffe chouffe 59 Jun 15 13:29 old_file

/dev/null

  • Discard output
command > /dev/null

I/O redirection

  • > file: writes standard output to file

  • 2>&1: writes standard error (2) to standard output (1)

  • > file 2>&1: writes standard output and error to file

  • &> out: writes standard error and standard output to out

  • Shell jargon example

grep "$*" <<EOF
command1
command2
command3
EOF

Built in Variables

Variable Description
$# Number of arguments
$* All arguments to shell
$@ All arguments to shell
$- Options supplied to shell
$? Return value of the last command executed
$$ Process ID of the shell
$! Process ID of the last command started with &

Signals

Signal Name Signal Number Description
SIGHUP 1 Hang up detected on controlling terminal or death of controlling process
SIGINT 2 Issued if the user sends an interrupt signal (Ctrl + C)
SIGQUIT 3 Issued if the user sends a quit signal (Ctrl + D)
SIGFPE 8 Issued if an illegal mathematical operation is attempted
SIGKILL 9 If a process gets this signal it must quit immediately and will not perform any clean-up operations
SIGALRM 14 Alarm clock signal (used for timers)
SIGTERM 15 Software termination signal (sent by kill by default) 
$ kill -l          # HUP INT QUIT ILL TRAP ABRT BUS FPE KILL USR1 SEGV USR2 PIPE ALRM TERM STKFLT CHLD CONT STOP TSTP TTIN TTOU URG XCPU XFSZ VTALRM PROF WINCH POLL PWR SYS
$ kill -l SIGHUP   # 1
$ kill -l 1        # SIGHUP
  • Send signals to a process
# kill -signal pid
$ kill -9 12345
$ kill -kill 12345
  • Set a trap which, on shell error or shell exit, deletes a temporary file xyz$$.
trap 'rm -f /tmp/xyz$$; exit' ERR EXIT
  • Set a trap which, on shell exit, kills the last command executed with &.
trap 'kill $!' EXIT
  • trap full example
#!/bin/sh

trap cleanup 1 2 3 6

cleanup()
{
  echo "Caught Signal ... cleaning up."
  rm -rf /tmp/temp_*.$$
  echo "Done cleanup ... quitting."
  exit 1
}

### main script
for i in *
do
  sed s/FOO/BAR/g $i > /tmp/temp_${i}.$$ && mv /tmp/temp_${i}.$$ $i
done