I want find out if a command exists on UNIX from a bash shell script. If command does not exists in $PATH, then I would like to display an error message on screen. How do I find out if a command exists on Posix system or not?
You can use the following commands:
- command command – You can execute a simple command or display information about commands with the command called command. This is recommend for all Posix systems.
- type command – This is recommend for bash user. The -P option force a PATH search for each COMMAND NAME, even if it is an alias, builtin, or function, and returns the name of the disk file that would be executed.
Posix Command Example
The syntax is as follows:
command -v command1 >/dev/null && echo "command1 Found In \$PATH" || echo "command1 Not Found in \$PATH"
#!/bin/bash # POSIX command lookup example CMDS="tar /usr/bin/mysqldump /path/to/other/command" for i in $CMDS do # command -v will return >0 when the $i is not found command -v $i >/dev/null && continue || { echo "$i command not found."; exit 1; } done # add rest of the script as we found all bins in $PATH echo "Starting backup...." |
type -P command Example
The syntax is as follows:
type -P command1 &>/dev/null && echo "Found" || echo "Not Found"
Here is a sample script:
#!/bin/bash # Use Bash builtin called type to determine whether a command exists or not # Use full path if possible... CMDS="tar /usr/bin/mysqldump ssh rsync" for i in $CMDS do type -P $i &>/dev/null && continue || { echo "$i command not found."; exit 1; } # Add rest of the script as we found all bins in $PATH echo "Starting backup...." /bin/tar .... |
You can use the exit status with which command as follows too:
which tar1 &>/dev/null [ $? -eq 0 ] || echo "tar1 command not found." |
I strongly recommend using full path for security reason for all sys admin activities. In this example, I’ve full path defined in /usr/local/nixcraft/redhat.paths.ini file:
_path_php_cgi="/usr/bin/php-cgi" _path_chroot="/usr/sbin/chroot" _path_lighttpd="/usr/sbin/lighttpd" _path_mknod="/bin/mknod" _path_spawn_fcgi="/usr/bin/spawn-fcgi" _path_cp="/bin/cp" _path_mv="/bin/mv" |
You can use the following code to verify that all path exists and than call those hard coded part later on:
#!/bin/bash source /usr/local/nixcraft/redhat.paths.ini # get those path vars paths=$(set | grep ^_path_*) # verify those paths for p in $paths do type -P ${p##*=} &>/dev/null || { echo "${p##*=} not found"; exit 1; } done # Alright, we got all binaries echo "Starting chroot() for ${_path_lighttpd}..." |
Here is a sample script which use type -P:
#!/bin/bash RRDTOOL=/usr/bin/rrdtool OUTDIR=/var/www/html/lighttpd.rrd INFILE=/home/lighttpd/rrd OUTPRE=lighttpd-traffic WIDTH=400 HEIGHT=100 # make sure rrdtool exists; else die with an error message type -P $RRDTOOL &>/dev/null || { echo "$RRDTOOL not found. Set \$RRDTOOL in $0"; exit 1; } # make sure dir exits; else create it [ -d $OUTDIR ] || mkdir -p $OUTDIR # make sure input file exits; else die with an error message [ -f $INFILE ] || { echo "$INFILE input file not found. set \$INFILE in $0"; exit 2; } DISP="-v bytes --title TrafficWebserver \ DEF:binraw=$INFILE:InOctets:AVERAGE \ DEF:binmaxraw=$INFILE:InOctets:MAX \ DEF:binminraw=$INFILE:InOctets:MIN \ DEF:bout=$INFILE:OutOctets:AVERAGE \ DEF:boutmax=$INFILE:OutOctets:MAX \ DEF:boutmin=$INFILE:OutOctets:MIN \ CDEF:bin=binraw,-1,* \ CDEF:binmax=binmaxraw,-1,* \ CDEF:binmin=binminraw,-1,* \ CDEF:binminmax=binmaxraw,binminraw,- \ CDEF:boutminmax=boutmax,boutmin,- \ AREA:binmin#ffffff: \ STACK:binmax#f00000: \ LINE1:binmin#a0a0a0: \ LINE1:binmax#a0a0a0: \ LINE2:bin#efb71d:incoming \ GPRINT:bin:MIN:%.2lf \ GPRINT:bin:AVERAGE:%.2lf \ GPRINT:bin:MAX:%.2lf \ AREA:boutmin#ffffff: \ STACK:boutminmax#00f000: \ LINE1:boutmin#a0a0a0: \ LINE1:boutmax#a0a0a0: \ LINE2:bout#a0a735:outgoing \ GPRINT:bout:MIN:%.2lf \ GPRINT:bout:AVERAGE:%.2lf \ GPRINT:bout:MAX:%.2lf \ " $RRDTOOL graph $OUTDIR/$OUTPRE-hour.png -a PNG --start -14400 $DISP -w $WIDTH -h $HEIGHT $RRDTOOL graph $OUTDIR/$OUTPRE-day.png -a PNG --start -86400 $DISP -w $WIDTH -h $HEIGHT $RRDTOOL graph $OUTDIR/$OUTPRE-month.png -a PNG --start -2592000 $DISP -w $WIDTH -h $HEIGHT OUTPRE=lighttpd-requests DISP="-v req --title RequestsperSecond -u 1 \ DEF:req=$INFILE:Requests:AVERAGE \ DEF:reqmax=$INFILE:Requests:MAX \ DEF:reqmin=$INFILE:Requests:MIN \ CDEF:reqminmax=reqmax,reqmin,- \ AREA:reqmin#ffffff: \ STACK:reqminmax#00f000: \ LINE1:reqmin#a0a0a0: \ LINE1:reqmax#a0a0a0: \ LINE2:req#00a735:requests" $RRDTOOL graph $OUTDIR/$OUTPRE-hour.png -a PNG --start -14400 $DISP -w $WIDTH -h $HEIGHT $RRDTOOL graph $OUTDIR/$OUTPRE-day.png -a PNG --start -86400 $DISP -w $WIDTH -h $HEIGHT $RRDTOOL graph $OUTDIR/$OUTPRE-month.png -a PNG --start -2592000 $DISP -w $WIDTH -h $HEIGHT |
Further readings:
Our Linux shell scripting tutorial and the following man pages:
man bash
man ksh
help type
help command
6 comment
use “which”
like “which somecommand”
where the result will be blank if it does not exist or the full path of the command if it does
Using “which” won’t show if the command that will be executed is an alias. “type” will show if there’s an alias.
According to The Open Group Base Specifications, `command -v` is not completely portable and `type -P` is not portable either:
type,
command
I have aliased ll=”ls -l” and it is there in $PATH. Yet “type -P” doesn’t show it. So @tonf is correct, which is simpler and easier to remember.
FWIW: `type -P` does not work with zsh:
% type -P mountpoint
zsh: bad option: -P
And “command” is not available with “ash”, a minimal shell used with BusyBox v1.16.1 (e.g. on a Synology Diskstation).
`type -P` appears to be specific to Bash.
`type -p` (lowercase “p”) works with bash, busybox/ash and zsh, and is similar to `-P`.