UNIX: Executing command using variable

UNIX for Beginners Q & A
1

Hi All,

I have one big script to execute multiple commands based on some condition and log the process.

I am not able to use | there, it is giving below error - refer code: it is small example.

Can we avoid this error ?

[alepo@a2e-rep-db tmp]$ cat test55.sh
#!/bin/bash
a="df -kh|head -2"
echo "$a"
$a
[alepo@a2e-rep-db tmp]$
[alepo@a2e-rep-db tmp]$ sh test55.sh
df -kh|head -2
df: invalid option -- '|'
Try 'df --help' for more information.
[alepo@a2e-rep-db tmp]$

This works fine.

[alepo@a2e-rep-db tmp]$ cat test55.sh
#!/bin/bash
a="df -kh"
echo "$a"
$a
[alepo@a2e-rep-db tmp]$
[alepo@a2e-rep-db tmp]$ sh test55.sh
df -kh
Filesystem      Size  Used Avail Use% Mounted on
devtmpfs         24G     0   24G   0% /dev
tmpfs            24G     0   24G   0% /dev/shm
tmpfs            24G   41M   24G   1% /run
tmpfs            24G     0   24G   0% /sys/fs/cgroup
/dev/vda1        99G   23G   72G  24% /
/dev/vdc1       6.9T  2.2T  4.5T  33% /data
tmpfs           4.7G     0  4.7G   0% /run/user/54321
tmpfs           4.7G     0  4.7G   0% /run/user/0
tmpfs           4.7G     0  4.7G   0% /run/user/1000
[alepo@a2e-rep-db tmp]$
2

Its failing on the pipe character at |. Try to escape it with a backslash like "df -kh\|head -2"

3

Hi Jake,

It gives another error.

[alepo@a2e-rep-db tmp]$ cat test55.sh
#!/bin/bash
a="df -kh\|head -2"
echo "$a"
$a
[alepo@a2e-rep-db tmp]$
[alepo@a2e-rep-db tmp]$ sh test55.sh
df -kh\|head -2
df: invalid option -- '\'
Try 'df --help' for more information.
[alepo@a2e-rep-db tmp]$
4

either:

#!/bin/bash
a=$(df -kh|head -2)
echo "$a"

OR

#!/bin/bash
a='df -kh|head -2'
echo "$a"
eval "$a"

1st is preferred.

2 Likes
5

Thank you Vgersh.

All these are small test.

[alepo@a2e-rep-db tmp]$ cat abc.txt
A1,df -kh|head -2
[alepo@a2e-rep-db tmp]$

[alepo@a2e-rep-db tmp]$ cat test55.sh
#!/bin/bash
IFS=$'\n'
for i in `cat abc.txt`
do
a=`awk -F"," '{print $2}' abc.txt`
eval "$a"
done
[alepo@a2e-rep-db tmp]$

[alepo@a2e-rep-db tmp]$ sh test55.sh
Filesystem      Size  Used Avail Use% Mounted on
devtmpfs         24G     0   24G   0% /dev
[alepo@a2e-rep-db tmp]$
6

or better yet /tmp/a.sh :

#!/bin/bash
while IFS=, read f1 f2 junk
do
   eval "$f2"
done < /tmp/a.txt

$ /tmp/a.sh
Filesystem      Size  Used Avail Use% Mounted on
C:/cygwin64     236G   98G  139G  42% /

OR (without the eval ):

#!/bin/bash
while IFS=, read f1 f2 junk
do
   echo "$f2"
done < /tmp/a.txt | bash
1 Like
7

Hi
Because the shell must first create a channel and then expand the variables.
And if it expands a variable, it skips the channel creation queue and tries to interpret the entire line as one command

1 Like